Diffusion bonding is an extremely valuable technique whereby two components are pressed together and heated so that the atoms in each component diffuse into the other component causing metal-to-metal bonding between the components. It is often required that the components are not bonded in all the contacting areas of the components and to prevent bonding it is known to apply a so-called `stopping-off` material to those areas of the components in which diffusion bonding is not desired.
Diffusion bonding can be combined with superplastic forming (SPF) which is a technique in which heated metal is subject to slow deformation during which the metal stretches and is thinned out in the deformed areas but does not neck or fracture. For example, combined superplastic forming/diffusion bonding processes are used for example to fabricate complex integrally stiffened panels by applying stopping-off material to defined areas of a stack of sheets, subjecting the stack to diffusion bonding so that the sheets bond together in those areas where stopping-off material has not been applied, clamping the stack into a mould, injecting an inert gas into the stack to form the stack superplastically into the shape of the mould, thereby forming the finished panel with the internal sheets of the stack forming the internal stiffening struts of the panel (see U.S. Pat. Nos. 3,834,000; 3,927,817 and 4,530,197).
Generally, stopping-off compositions comprise a ceramic material in a suitable vehicle, e.g. a volatile liquid, so that the stop-off composition can be applied to the components e.g. by screen-printing.
Various stopping-off compositions have been described in the prior art as follows:
EP-A-0 266 073 describes a composition consisting of yttria suspended in a solution of a thermoplastic acrylic polymer together with an anti-settling or anti-coagulant additive. Such a composition can be applied by screen-printing.
U.S. Pat. No. 3,638,298 describes a composition in the form of an aqueous slurry of calcium oxide which can be applied by brushing or dipping; the calcium oxide can be replaced by yttria (when a calcium silicate binder may also be incorporated into the slurry) or other rare earth oxide or cerium sulphide.
U.S. Pat. No. 3,834,000 describes the use of flake graphite, molybdenum disulphide, silica chromic oxide and alumina in pre-hydrolysed ethyl silicate as a stopping-off composition which can be applied by brushing, rolling, or spraying or by means of a doctor blade to areas not covered by a mask.
U.S. Pat. No. 3,906,617 describes a stopping-off composition for preventing brazing and diffusion bonding; the composition contains alumina, titania, magnesia or rare earth compounds in a fugitive carrier which can be applied by brushing.
U.S. Pat. No. 3,927,817 describes a stopping-off composition to prevent diffusion bonding which contains graphite, boron nitride or yttria in a binder; such a composition is applied by spraying.
U.S. Pat. No. 4,220,276 states that an improved stopping-off composition for combined superplastic forming and diffusion bonding techniques is provided by yttria having a particle size of greater than 5 microns and an inert organic liquid vehicle which includes a volatile binder that evaporates at diffusion bonding temperature leaving no residue.
U.S. Pat. No. 4,530,197 describes a stopping-off composition in the form of graphite, boron nitride or yttria in a suitable binder which can be applied to selected areas by screen printing.
Because of the low density, high strength and relative cheapness of aluminum, it is ideal for use in fabricating aeroplane parts but unfortunately its tenaceous oxide has prevented its wide-spread use in diffusion bonding processes since the oxide blocks metal diffusion and so diffusion bonding cannot take place. It has been proposed in EP-0 350 220 to remove the oxide layer prior to diffusion bonding and such a process is feasible so long as the diffusion bonding is performed within about 20 minutes of the oxide removal. However, the volatile carrier liquid used in the above-described known stopping-off compositions, which are generally used in diffusion bonding of titanium and its alloys (e.g. Ti A16 V4), contaminate the unstopped-off areas and prevent diffusion bonding even in areas where the oxide has been removed.